The present invention relates to a corona discharging device having a corona discharging electrode to which a high voltage is applied so that corona discharge takes place, and more particularly to a corona discharging device which is usable with an electrophotographic apparatus such as an electrophotographic copying machine and a laser beam printer and which is actable on a photosensitive member to electrically charge the same.
A corona discharging device wherein the corona discharge is effected by applying a high voltage to a corona discharging electrode, is divided into three categories from the standpoint of the voltage applied thereto.
In the first category, a DC voltage is applied to the corona discharging electrode. A DC high voltage of positive polarity is applied to produce a positive corona, while a negative high DC voltage is applied to produce a negative corona.
In the second category, an AC voltage is applied to the corona discharging electrode, whereby both positive and negative coronas are produced by the corona discharging electrode.
In the third, an AC voltage and a DC voltage which are superposed with each other are applied to the corona discharging electrode. In this case, the main part is the AC voltage producing the positive and negative corona. The difference in the corona discharge (current) between the two components is effective to discharge the surface to be charged. Here, the DC voltage is auxiliary and is effective to control the difference in the corona discharge current between the negative component and positive component, or to maintain the difference constant.
When a corona discharging device is used for charging to a desired potential a member to be charged such as a photosensitive member, the first category discharger is most frequently used. However, when a DC voltage is applied to the corona discharging electrode to produce a negative corona, non-uniform discharge takes place unless the corona discharging current from the corona discharging electrode is significantly increased. Therefore, with the view to increasing the corona discharging current, it is necessary to increase the output and the capacity of the high voltage power source, resulting in a large size high voltage source. The increase of the discharging current involves an additional drawback that the corona products such as ozone and nitrogen oxide are also increased. The problems described above arise also in the case of positive corona discharge, although they are relatively less as compared with the negative corona.
It would be considered that the third category device is used to effect the charging. However, since the charging action is provided by the difference in the corona discharging current between the positive and negative components in this device, the charging efficiency is low when the surface is charged to a certain polarity.